The invention relates to electrostatic copiers. More particularly, the invention relates to the optical system for electrostatic copiers which include a copying drum that has a lateral guide mechanism for the paper and which includes a primary deviating mirror which reflects the light from the line region of an original which is traversed along the length of the paper. The image of the illuminated part of the original is focused by suitable optical means on the copying drum which rotates synchronously with the motion of the illuminated part of the original.
In known electrostatic copiers of the above described type, the original page is placed on a glass plate and is illuminated line by line. The optical system which consists substantially of the deviating mirror and the objective lens or mirror focuses the image of the temporarily illuminated line portion of the original onto the electrostatically charged copying drum. Subsequently the paper on which the copy is to be made is brought into juxtaposition to the copying drum and is guided along the edge of the drum by a suitable margin guide, for example a guide ribbon. In order to permit copying the entirety of the original onto the copying drum, the latter is rotated synchronously with the motion of the illuminating mechanism of the original and at the same linear speed. The traveling illumination of the line portions of the original may be performed either by having a fixed lamp over which the original document is passed or by placing the lamp on a movable sled which is moved over the original document. In the latter case it is required to include a movable mirror system in order to keep the optical path from the object to the copying drum constant so as to maintain the required focusing.
It is a disadvantage of these known copying machines that they are basically suitable only for copying with a fixed reproduction format. Normally the optical system is so designed as to produce, for example, an image in the ratio of one-to-one whose lateral edge coincides with the paper guide of the copying drum. If the optical system is altered, for example by displacing the objective lens or mirror so as to obtain a reduced image, the reduced image of the original will normally be located in the central region of copying drum and will no longer be contiguous to the lateral paper guide. In order to place the image at the edge, i.e. the lateral guide of the copying drum, the guide mechanism would have to be displaced on the drum which would entail substantial technical difficulties.
When known copying machines were heretofore used with the provision of variable reproduction format, it has therefore been required to use very expensive exchangeable optical systems or else to make available a different copier for each desired reproduction format.